1. Field of the Invention
The present invention relates to a fixed-point detector for detecting a signal from light diffracted by a diffraction grating and then obtaining a fixed point on the basis of the signal, and also relates to a displacement-measuring apparatus provided with such a fixed-point detector.
2. Description of the Related Art
Heretofore, a displacement-measuring apparatus having a scale and a detection head has been known as a measuring apparatus for carrying out a precision measurement of linear displacement, rotational displacement, or the like. Such a displacement-measuring apparatus has been widely used for an apparatus for mounting electric parts, in which high-precision positioning control of a product being fed may be required, a measuring apparatus for measuring the dimensions of a component, or the like. Such a displacement-measuring apparatus is provided with a fixed-point detector and a base position for the measurement or fixed point (starting point) has been defined.
Japanese Unexamined Patent Application Publication No. 04-324316 describes an example of the related-art fixed-point detector. Specifically, the Patent Document descries a fixed-point detector suitably installed in a linear encoder, a rotary encoder, or the like by which the position of an object can be optically detected. The fixed-point detector described in the above Patent Document includes “a pair of diffraction gratings, with which diffraction efficiencies of incident light vary in the measurement direction, each dividing light from a single light source into two light beams; a photoelectric converter converting the light beams diffracted from the respective diffraction gratings into electric signals; and a detector detecting that the levels of the electric signals being converted are coincident with each other”.
In the fixed-point detector described in the above-described Patent Document, light beams diffracted from the pair of diffraction gratings, with which diffraction efficiencies vary in the measurement direction, are converted by two photoelectric converters into electric signals, respectively. Subsequently, a position at which levels of two electric signals are coincident with each other is defined as a specified point.